It is simply a designation placed for a pathogen that doctors and scientists will not know how to react to. It also represents a disease that could be started by a biological mutation.

"Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease," WHO says on their website.

They hope this keeps public health officials on their toes and ready for a potential threat to society even if it isn't predictable or hasn't been seen before, according to The New York Post.

Other diseases on the list actually exist at the moment and are commonly known such as: Ebola, Marburg, Rift Valley fever, Zika, Lassa fever, Crimean-Congo haemorrhagic fever, Nipah and henipaviral diseases.